emmy0love, unfortunately we can not control what our parents do. Freaking out, isn't going to help the situation at all. Have you tried talking to your mom about your feelings? This is most certainly where you need to start.
You may want to give some more details, like how old you are, if you like or dislike the guy, etc. Something to give us a better idea of why this is even a question, as the obvious answer is go congratulate her and celebrate.
What do you mean.... What do you do? I dont think YOU need to do anything. If you are supportive of your mother... then let her know that. If this guy makes her happy.... then shouldnt that matter? I would think you would want her to be happy.